1. Field of the Invention
The present invention relates to a photosensitive resin composition for forming a laser engravable printing element. More particularly, the present invention is concerned with a photosensitive resin composition for forming a laser engravable printing element, comprising: (a) a resin which is in a solid state at 20° C., wherein the resin has a number average molecular weight of from 5,000 to 300,000, (b) an organic compound having a number average molecular weight of less than 5,000 and having at least one polymerizable unsaturated group per molecule, and (c) an inorganic porous material having an average pore diameter of from 1 nm to 1,000 nm, a pore volume of from 0.1 ml/g to 10 ml/g and a number average particle diameter of not more than 10 μm. Further, the present invention is also concerned with a laser engravable printing element formed from the photosensitive resin composition of the present invention. By the use of the photosensitive resin composition of the present invention, it becomes possible to obtain a printing element which can suppress the generation of debris during the laser engraving thereof, thereby rendering easy the removal of debris. Further, the obtained printing element is advantageous in that a precise image can be formed on the printing element by laser engraving, and in that the resultant image-bearing printing plate not only has small surface tack and excellent abrasion resistance, but also is capable of suppressing the adherence of paper dust and the like to the printing element and the occurrence of printing defects. Further, the present invention is also concerned with a method for producing a laser engravable printing element using the photosensitive resin composition of the present invention.
2. Prior Art
The flexographic printing method is used in the production of packaging materials (such as a cardboard, a paperware, a paper bag and a flexible packaging film) and materials for construction and furnishing (such as a wall paper and an ornamental board) and also used for printing labels. Such flexographic printing method has been increasing its importance among other printing methods. A photosensitive resin is generally employed for producing a flexographic printing plate, and the production of a flexographic printing plate using a photosensitive resin has conventionally been performed by the following method. A photo-mask bearing a pattern is placed on a liquid resin or a solid resin sheet (obtained by molding a resin into a sheet), and the resultant masked resin is imagewise exposed to light, to thereby crosslink the exposed portions of the resin, followed by developing treatment in which the unexposed portions of the resin (i.e., uncrosslinked resin portions) are washed away with a developing liquid. Recently, the so-called “flexo CTP (Computer to Plate) method” has been developed. In this method, a thin, light absorption layer called “black layer” is formed on the surface of a photosensitive resin plate, and the resultant resin plate is irradiated with a laser to ablate (evaporate) desired portions of the black layer to form a mask bearing an image (formed by the unablated portions of the black layer) on the resin plate directly without separately preparing a mask. Subsequently, the resultant resin plate is imagewise exposed to light through the mask, to thereby crosslink the exposed portions of the resin, followed by developing treatment in which the unexposed portions of the resin (i.e., uncrosslinked resin portions) are washed away with a developing liquid. Since the efficiency in producing the printing plates has been improved by this method, its use is beginning to expand in a wide variety of fields. However, this method also requires a developing treatment as in the case of other methods and, hence, the improvement in the efficiency in producing the printing plates is limited. Therefore, it has been desired to develop a method for forming a relief pattern directly on a printing element by using a laser without a need for a developing treatment.
As an example of a method for producing a printing plate by directly forming a relief pattern on a printing element using a laser, which method does not require a developing treatment, there can be mentioned a method in which a printing element is engraved directly with a laser. Such a method has already been used for producing relief plates and stamps, in which various materials are used for forming the printing elements.
For example, U.S. Pat. No. 3,549,733 discloses the use of a polyoxymethylene or polychloral for forming a printing element. Further, Japanese Patent Application prior-to-examination Publication (Tokuhyo) No. Hei 10-512823 (corresponding to DE 19625749 A) describes the use of a silicone polymer or a silicone fluoropolymer for forming a printing element. In each of the specific examples of compositions used for forming the printing element, which are described in this patent document, fillers, such as amorphous silica, are added to the above-mentioned polymer. However, a photosensitive resin is not used in the inventions disclosed in the above-mentioned patent documents. In the above-mentioned Japanese Patent Application prior-to-examination Publication (Tokuhyo) No. Hei 10-512823, amorphous silica is added to the polymer for improving the mechanical properties of the polymer and reducing the amount of an expensive elastomer used in the printing element. Further, this patent document has no description about the properties of the amorphous silica used.
Unexamined Japanese Patent Application Laid-Open Specification No. 2001-121833 (corresponding to EP 1080883 A) describes the use of a mixture of a silicone rubber and carbon black for producing a printing element, wherein the carbon black is used as a laser beam absorber. However, a photosensitive resin is not used in this invention.
Unexamined Japanese Patent Application Laid-Open Specification No. 2001-328365 discloses the use of a graft-copolymer as a material for producing a printing element. Further, this patent document describes that, for improving the mechanical properties of the graft copolymer, a non-porous silica having a particle diameter which is smaller than the wavelength of the visible light may be mixed with the graft copolymer. However, this patent document has no description about the removal of a liquid debris which is generated by laser engraving.
Unexamined Japanese Patent Application Laid-Open Specification No. 2002-3665 uses an elastomer composed mainly of ethylene monomer units, and this patent document describes that silica may be added to the elastomer as a reinforcing agent. In the Working Examples of this patent document, 50 parts by weight of a porous silica and 50 parts by weight of calcium carbonate were added to 100 parts by weight of a resin. Both of the above-mentioned porous silica and calcium carbonate were used only as white reinforcing agents and, for achieving a satisfactory reinforcing effect, those reinforcing agents were used in large amounts (total amount of the reinforcing agents was as large as 100 parts by weight). That is, the use of a silica in this patent document does not extend beyond the customary technology in which a silica is used as a reinforcing agent for a rubber. Further, the resin used in this patent document is not a photosensitive resin and the resin is cured by heating. Therefore, the curing rate of the resin is low and the dimensional precision of a sheet obtained from the resin is poor.
Each of Japanese Patent No. 2846954 (corresponding to U.S. Pat. No. 5,798,202) and Japanese Patent No. 2846955 (corresponding to U.S. Pat. No. 5,804,353) discloses the use of a reinforced elastomer material obtained by mechanically, photochemically and thermochemically reinforcing a thermoplastic elastomer, such as SBS (polystyrene-polybutadiene-polystyrene), SIS (polystyrene-polyisoprene-polystyrene) and SEBS (polystyrene-polyethylene/polybutadiene-polystyrene). When a printing element formed from a thermoplastic elastomer is engraved with a laser beam having an oscillation wavelength within the infrared region, even portions of the printing element which are distant from the portion irradiated with the laser beam also tend to melt by heat. Therefore, the resultant printing element cannot be used for preparing an engraved pattern having a high resolution. For removing this problem, it is necessary to add a filler to the thermoplastic elastomer to thereby improve the mechanical properties thereof. In each of the above-mentioned patent documents, for improving the mechanical properties of the thermoplastic elastomer and increasing the absorption of the laser beam by the thermoplastic elastomer, carbon black having excellent ability to enhance the mechanical properties of a resin is added to a thermoplastic elastomer. However, since carbon black is added to the elastomer, light transmittance of the elastomer is lowered, which is disadvantageous when it is attempted to crosslink the elastomer by irradiation (i.e., when it is attempted to perform a photochemical reinforcement of the elastomer). Therefore, when the above-mentioned reinforced elastomer material is subjected to laser engraving, it results in the generation of a large amount of debris (including viscous liquid material) which is difficult to remove. The generation of such debris not only necessitates a time-consuming treatment for removing the debris, but also causes problems, such as an imprecise boundary between elastomer portions which have been melted by laser beam irradiation and unmolten elastomer portions which form the relief pattern, the swelling of the edges of the unmolten elastomer portions forming the relief pattern, the adherence of the molten elastomer to the surfaces and/or sides of the unmolten elastomer portions forming the relief pattern, and the destruction of portions of the relief pattern which correspond to the dots of a print obtained using the relief pattern.
Further, when a large amount of liquid debris, which is presumed to be a laser decomposition product of the resin, is generated during the laser engraving of the printing element, the liquid debris stains the optical parts of a laser engraving apparatus. When the liquid debris is adhered to the surface of optical parts, such as a lens and a mirror, the resin causes serious troubles of the apparatus, such as burnout of the apparatus.
In the above-mentioned reinforced elastomer materials disclosed in Japanese Patent Nos. 2846954 and 2846955, a filler, such as carbon black, inhibits the reinforced elastomer materials from being completely photocured. Therefore, when the reinforced elastomer materials are used for forming a printing element, the formed printing element suffers problems, such as unsatisfactory engraving depth and generation of viscous debris. For solving these problems, Unexamined Japanese Patent Application Laid-Open Specification No. 2002-244289 discloses the use of a thermoplastic elastomer composition obtained by adding to a thermoplastic elastomer a bleachable compound as a photopolymerization initiator and further adding an additive having a functional group (e.g., an Si—O group) which absorbs infrared radiation, to thereby produce a printing element having improved engraving sensitivity (i.e., index defined as an engraving depth per unit time). A bleachable photopolymerization initiator (such as triphenylphosphine oxide) generates radical species while being decomposed by absorbing light. Simultaneously with the decomposition of the bleachable photopolymerization initiator, the bleachable photopolymerization initiator loses its capacity to absorb radiation. Therefore, when a printing element is produced using a photosensitive resin composition containing a bleachable photopolymerization initiator, the light transmittance into the inner portion of the photosensitive resin composition is improved and the photosensitive resin composition can be cured satisfactorily, thereby suppressing the generation of liquid debris. In the Working Examples of the above-mentioned patent document, an additive, such as zirconium silicate (ZrSiO4) or amorphous silica, is used, but there is no description about the properties of the additive used. As a most preferred example of a photosensitive resin composition having excellent engraving sensitivity and high engraving debris cleanability (i.e., efficiency in removing debris generated during the laser engraving), there is mentioned a resin composition containing a bleachable photopolymerization initiator and zirconium silicate in combination. In a working example of the above-mentioned patent document which uses an amorphous silica instead of zirconium silicate, it is described that debris generated during the laser engraving was slightly tacky and the cleaning of debris was not so difficult. Further, a combination of 2,2-dimethoxy-2-phenylacetophenone (which is generally used as a photopolymerization initiator for a photosensitive resin composition) and zirconium silicate is described in a Comparative Example of the above-mentioned patent document.
The above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. 2002-244289 contains no detailed description about the type and properties of the zirconium silicate used. Zirconium silicate is a crystalline inorganic compound having a high melting point, and it is very difficult to produce porous microparticles of amorphous zirconium silicate by any of the melt method, the wet method, the sol-gel method and the like, while maintaining the composition of zirconium silicate (theoretical chemical composition of this compound ZrSiO4: 64.0% of ZrO2 and 34.0% of SiO2). Therefore, microparticles of zirconium silicate are obtained by pulverizing a bulk of crystals, and it is presumed that the particles obtained in such a manner are not porous. In “Kagaku Dai Jiten (Encyclopedia Chimica)” published by KYORITSU SHUPPAN CO., LTD., Japan, it is described that zirconium silicate, which is a mineral silicate of zirconium, is the main component of a mineral known as zircon, and that, in many cases, zirconium silicate is in the form of short prismatic crystals having chemical and physical properties which are greatly different from those of zirconium oxide. The above-mentioned document describes that the term “mineral” used therein means a homogeneous inorganic substance which is a component of the earth's crust and has a crystal structure in which atoms and ions are regularly arranged. In addition, in “13901 no Kagaku Shohin (13901 Chemical Products)” published by The Chemical Daily Co., Ltd, Japan, it is also described that pulverized zirconium sand is called “zirconium silicate” in an open market. The present inventors analyzed a commercially available zirconium silicate (Product No. 261-00515 (catalogue issued in 2002); manufactured and sold by Wako Pure Chemical Industries, Ltd., Japan). Specifically, the observation of the zirconium silicate particles under a scanning electron microscope revealed that the particles have no definite shape. Further, the pore volume of the zirconium silicate particles measured by the nitrogen adsorption method was as small as 0.026 ml/g. Thus, the present inventors found that the above-mentioned commercially available zirconium silicate was not porous. In addition, another commercially available zirconium silicate (Product No. 38328-7; manufactured and sold by Sigma-Aldrich Co., U.S.A.) was also analyzed in the above-mentioned manner, and it was confirmed that this zirconium silicate was also not porous.
Furthermore, in the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. 2002-244289, there is no description about the relationship between the engraving debris cleanability and the properties of the particles used as an additive. In addition, there is no description about the preferred shape of the particles used as the additive. Therefore, it is apparent that the invention disclosed in this patent document is based on a technical concept that the generation of liquid debris is lowered by improving the light transmittance into the inner portion of the photosensitive resin composition to thereby satisfactorily cure the photosensitive resin composition. Thus, although the debris cleaning effect is reported in this patent document, this effect has no relation to the ability of an inorganic porous material to remove a liquid debris.